If you log into your self-serve account you can check the total usage per billing cycle. If you don't see any data mentioned, you have used it all up for this billing cycle.
You can buy a data add-on to tide you over 'til your renewal date. (The 200MB and 1GB data add-ons don't expire until you use them all up and will only be deducted once your plan's data is all used up. Be aware that it's a two step process to buy it.)
Some members have also successfully contacted the moderators to get help with renewing their plan early.
You can use a data managing app to monitor your use. If you are on a 90 day billing cycle, an app like Data Witness or My Data Manager can be helpful (ability to monitor full 90 days).
Turn the background data usage of your apps off and make sure that OS or app updates are only downloaded on WiFi.
If you are using an Iphone ensure that WiFi assist is turned off since it can be eating into your data allotment.
More info on data usage: https://productioncommunity.publicmobile.ca/t5/Knowledge-Base/Managing-your-data-usage/ta-p/152760
